11. Electro-desalination of Buildings Suffering from Salt Weathering

Salt-induced decay is a threat to many historic and recent stone and brick buildings and monuments. The salts cause, e.g., surface scaling or paint peeling. There is a need for development of reliable methods to remove the damaging salts in order to stop the decay and not to lose important heritage. Electro-desalination of porous materials has been tested in laboratory scale with success. This method has the strength of not being very dependent on the characteristics of the material to be desalinated. Chlorides, nitrates, and sulfates can be removed at high transference numbers. The removal of SO₄ ²⁻ is though slower than the two other anions as the theoretical ionic mobility is lower and because gypsum is formed by precipitation with Ca²⁺ ions in the pore solution. Another important strength of electro-desalination is that the electrodes can be placed around the most decayed and fragile surfaces thus not causing additional physical damage to these surfaces. For electro-desalination it is important that the water content is sufficient for salts to be dissolved and not nucleated, but as salt decay is caused by repeated phase changes between dissolved and nucleated, the salts must be dissolved at some points. A final proof of concept for electro-desalination is still lacking, as full desalination has not yet been carried out in pilot scale. Different pilot scale experiments have been conducted, but they were all stopped before full desalination. However, when they were stopped, the transference number for the target ion was still high, and the desalination could have progressed further.